Glycogen Metabolism

Question 1

What are the 2 major sites of glycogen storage ?

Answer 1

1. Liver- maintains blood glucose levels
2. Muscle- provides local energy for muscle contraction

Question 2

What proteins facilitate glucose uptake from the blood ?

Answer 2

1. GLUT 2- present in liver and B cells of pancreas. Low affinity, high capacity
2. GLUT 4- present in muscle and fat cells. It is insulin dependent

Question 3

What is glycogenesis ?

Answer 3

The process of synthesising glycogen- the storage form of glucose

Question 4

What are the steps of glycogenesis ?

Answer 4

1. Glucose is converted into Glucose 6-phosphate by hexokinase in muscle or glucokinase in the liver
2. Glucose 6-phosphate is converted into Glucose 1-phosphate by Phosphoglucomutase
3. Glucose 1-phosphate is activated by enzyme UDP-glucose pyrophosphorylase, which produces UDP-glucose
4. Glycogenin auto catalyses the addition of glucose to itself to start the glycogen chain.
5. UDP-glucose is added to glycogen chain to form alpha 1,4 glycosidic bonds using enzyme glycogen synthase

Question 5

What is the key regulatory enzyme of glycogen synthesis?

Answer 5

Glycogen synthase
Active form: desphosphorylated (stimulated by insulin)
Inactive form: phosphorylated

Question 6

What forms alpha 1,6 glycosidic bonds in glycogenesis ?

Answer 6

A branching enzyme

Question 7

Describe the steps of glycogenolysis

Answer 7

1. Glycogen phosphorylase cleaves alpha 1,4 glycosidic bonds by adding inorganic phosphate to produce Glucose-1-phosphate
2. A debranching enzyme removes the 1,6 glycosidic bonds
3. Glucose 1-phosphate is converted into Glucose 6-phosphate using enzyme Phosphoglucomutase
4. In the liver Glucose 6-phosphatase breaks Glucose 6-phosphate into glucose which can enter the blood

Question 8

What is the key regulatory enzyme in glucose breakdown ?

Answer 8

Glycogen phosphorylase

Question 9

How are glucose synthesis and breakdown regulated ?

Answer 9

By hormone triggered (glucagon or adrenaline) cAMP cascade acting through Protein kinase A

Question 10

What are examples of glycogen storage diseases ?

Answer 10

1. Von Gierke’s disease
2. Cori’s disease
3. McArlde’s disease

Question 11

What is Von Gierke’s disease ?

Answer 11

Type 1.
Glucose-6-phosphatase is missing
Hypoglycaemia occurs

Question 12

What is Cori’s disease ?

Answer 12

Type 3
Deficiency of Glycogen debranching enzyme.

Question 13

What is McArlde’s disease ?

Answer 13

Type 5
Muscle phosphorylase missing
Muscle cramps occur

Question 14

What does protein phosphatase 1 do ?

Answer 14

Reverses the effect of protein kinase A